In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), standardization of techniques for improving deterioration of communication quality due to recent sharp increase in mobile traffic and for achieving faster communication has been performed. Further, standardization of techniques for avoiding increase in a control signaling load due to connections of an enormous number of Machine to Machine (M2M) terminals to an LTE network has been performed (Non-Patent Literature 1). The M2M terminals are, for example, terminals that perform communication without human intervention. The M2M terminals are placed in various types of equipment including machines (e.g., vending machines, gas meters, electric meters, vehicles, railway vehicles, and ships) and sensors (e.g., environmental, agricultural, and traffic sensors). In the LTE, communication performed by the M2M terminals are referred to as Machine Type Communications (MTC) and a terminal performing the MTC is referred to as an MTC terminal (MTC User Equipment (MTC UE)).
While M2M service providers need to distribute an enormous number of M2M terminals, there is a limit to the cost allowable for each M2M terminal. Therefore, it is required M2M terminals be implemented at a low cost, and M2M terminals be able to perform communication with low power consumption, for example. Further, in one use case, MTC UEs perform communication while they are fixedly or statically installed in buildings. In this case, the radio quality of MTC UEs may be always low and accordingly coverage enhancement technique is especially needed for MTC devices compared to normal UEs having mobility (e.g., mobile telephones, smartphones, tablet computers, and notebook personal computers (notebook PCs)). Further, functional restrictions contributing to reduction of the cost include, for example, a low maximum transmission power, a small number of reception antennas, no support of high-order modulation schemes (e.g., 64 Quadrature Amplitude Modulation (64 QAM)), and a narrow operating bandwidth (e.g., 1.25 MHz), which lower the maximum transmission rate of MTC UEs. Therefore, in the LTE, standardization of techniques for enhancing communication characteristics of MTC UEs (i.e., coverage), which are expected to be lower than those of normal UE has been performed (Non-Patent Literature 2). In the following description, some examples of the techniques for enhancing coverage of MTC UEs discussed in the LTE are described. It can be said that the coverage enhancement techniques (coverage enhancement processing) for MTC UEs described below are processing for enhancing or improving communication characteristics or communication quality of MTC UEs. The state of a UE to which these special coverage enhancement techniques has been applied is referred to as a coverage enhancement mode (Enhanced Coverage Mode (ECM)).
The ECM can improve, for example, a reception characteristic of a Physical Broadcast Channel (PBCH), a transmission characteristic of a Physical Random Access Channel (PRACH) preamble (i.e., detection characteristic in an eNB), a reception characteristic of a Physical Downlink Shared Channel (PDSCH), and a transmission characteristic of a Physical Uplink Shared Channel (PUSCH). The PBCH is a downlink broadcast channel used by an eNB to transmit broadcast information commonly used within a cell. The PRACH is an uplink physical channel used by a UE for an initial access to a radio base station (eNB). The PDSCH is a downlink physical channel used for data reception by a UE. The PUSCH is an uplink physical channel used for data transmission by a UE.
One processing that is being discussed to improve a reception characteristic of the PBCH is to repeatedly transmit broadcast information on the PBCH a number of extra times as compared to the normal operation by a predetermined number of times (Non-Patent Literature 3). One processing that is being discussed to improve a transmission characteristic of the PRACH is to repeatedly transmit the PRACH (i.e., preamble) a predetermined number of times (Non-Patent Literature 4). Further, one processing that is being discussed to improve a reception characteristic of the PDSCH and a transmission characteristic of the PUSCH is to repeatedly transmit the PDSCH and the PUSCH over multiple subframes (Non-Patent Literature 5). According to the above processing, communication characteristics of MTC UEs that is expected to be lower than that of normal UEs will be improved.
It is assumed that the coverage enhancement processing in the ECM is executed by MTC UEs that perform a delay tolerant access. The delay tolerant access is defined as being a new EstablishmentCause that is specified in an RRC Connection Request message and is used, for example, to control an overload. The delay tolerant access is mainly intended for MTC UEs that execute a delay-tolerant MTC application. For example, in a metering service (meter reading service), there is no need to send a metering report to a remote system in real time (or in exact communication cycles) and a long delay may be allowed for the transmission of the metering report. When an eNB imposes overload control on the delay tolerant access, the eNB may reject an RRC Connection Request transmitted by a RRC Connection Request message that contains “EstablishmentCause” indicating the delay tolerant access.